1. Field of the Invention
The present invention relates generally to a mobile communications system for carrying out communications according to a polling system carried out by one master station between a plurality of mobile stations via a plurality of base stations.
2. Description of the Prior Art
In recent years, unmanned work mechanization has been increasing in a variety of fields and in particular many factory operations are being automated. In response to requests for such factory automation, an unmanned vehicle system has been developed for automatically carrying out operations such as transportation of goods by unmanned vehicles. In this unmanned vehicle system a master station is provided for managing a plurality of unmanned vehicles, mobile unit radio communication is carried out between this master station and the unmanned vehicles, and the movement of the unmanned vehicles is thus controlled.
A variety of communications systems exist for this type of mobile unit radio communication, one being an IEC system based on Draft SC65C of the International Electrotechnical Commission. In this IEC system, a plurality of base stations are connected by cables to a master station and radio communication is carried out by a polling system (system whereby mobile stations execute transmission at a timing specified by the master station) between a plurality of base stations and a plurality of mobile stations. In addition, in this system the mobile stations managed by one base station are allocated in advance and all base stations carry out polling in a parallel operation, thus speeding up response time.
However, the abovementioned prior art has the problem that the possible mobility range of the mobile stations is restricted since the mobile stations to be managed by the base stations are determined in advance. In other words, if base stations are positioned at a distance from one another the range of manageable mobile stations of the entire system increases while, if the mobile stations managed by each base station are fixed, communication will not be possible when a mobile station moves away from the base station which has jurisdiction over it. In order to increase the level of freedom of the system it would be better if the mobile stations managed by the base stations were not fixed.
In Japanese Patent Application No. Hei 6-202175 the present applicant presented a system for carrying out replacement of a base station wherein a mobile station determines the base station to which it should transmit. According to that system, replacement of a base station with jurisdiction to accompany the movement of a mobile station could be smoothly carried out.
However, depending on the system, there are cases in which the number of mobile stations becomes considerable and a multiplicity of mobile stations is concentrated in the jurisdiction area of a single base station. Hence the problems that since communication via a base station is carried out in sequence to the mobile stations over which that base station has jurisdiction, the incidence of the communications diminishes and the time taken for a single communication to make one tour (namely the time taken to complete polling communication with all the mobile stations) increases.
Therefore it is envisaged that when mobile stations are concentrated at one base station, jurisdiction over these mobile stations may be possible at another base station and, in a case in which the relevant other base station is serving only a small number of mobile stations, it would be preferable to transfer a portion of the mobile stations to the jurisdiction of this other base station.